Electroradiographic x-ray sensitive element containing tetragonal lead monoxide



United States Patent 3,543,025 ELECTRORADIOGRAPHIC X-RAY SENSI- TIVE ELEMENT CONTAINING TETRA- GONAL LEAD MONOXIDE Oris L. Stanton, Rochester, N.Y., assignor to Eastman Kodak Company, Rochester, N.Y., a corporation of New Jersey No Drawing. Filed Nov. 12, 1968, Ser. No. 775,193 Int. Cl. G03b 41/16 US. Cl. 250-65 20 Claims ABSTRACT OF THE DISCLOSURE Electroradiographic elements containing, as a photoconductor, tetragonal lead monoxide prepared by heating orthorhombic lead monoxide in an aqueous solution containing carbonate ions and subjecting the material to a post heat treatment are sensitive to X-radiation and useful in electroradiography.

This invention relates to electroradiography, and in particular to photoconductive compositions and elements.

The process of xeroradiography employs xeroradiographic element comprising a support material bearing a coating of a normally insulating material whose elec trical resistance varies with the amount of incident X-radiation it receives during an imagewise exposure. The element, commonly termed a photoconductive element, is first given a uniform surface charge. It is then exposed to a pattern of X-radiation which has the efiiect of differentially reducing the potential of this surface charge in accordance with the relative energy contained in various parts of the radiation pattern. The differential surface charge or electrostatic latent image remaining on the xeroradiographic element is then made visible by contacting the surface With a suitable electroscopic marking material. Such marking material or toner, whether contained in an insulating liquid or on a dry carrier, can be deposited on the exposed surface in accordance with either the charge pattern or in the absence of charge pattern as desired. Deposited marking material can then be either permanently fixed to the surface of the sensitive element by known means such as heat, pressure, solvent vapor, or the like, or transferred to a second element to which it can similarly be fixed. Likewise, the electrostatic latent image can be transferred to a second element and developed there.

Various photoconductive insulating materials have been employed in the manufacture of xeroradiographic elements. For example, inorganic materials such as amorphous selenium, cadmium sulfide, zinc sulfide and sulfur, and organic materials such as anthracene and stilbene coated on a suitable support are sensitive to X-rays.

Another material which has been found to be sensitive to X-radiation is orthorhombic lead monoxide. One of the problems encountered in using any of these materials is their exposure-speed characteristics. Most of these photoconductors are too slow for medical applications. Others, such as selenium and orthorhombic lead monoxide have adequate speeds for exposures of such areas as the limbs, hips, shoulders, cervical spine and ribs but are not fast enough for heavier parts such as the abdomen, pelvis and lumbar spine. Because of the slow speeds involved in the exposures of the members of this latter group, these photoconductors are unsuitable since, in order to obtain acceptable reproductions, the member would have to be subjected to X-rays for an undesirably long period.

3,543,025 Patented Nov. 24, 1970 In an application Ser. No. 775,195 entitled X-Ray Sensitive Electroradiographic Elements, by R. F. Reithel, filed concurrently herewith, electroradiographic elements having improved exposure speeds in the X-ray region are described. These elements contain, as photoconductors, red tetragonal lead monoxide prepared by heating particulate yellow orthorhombic lead monoxide in water for 30 minutes or more and subjecting the separated material to a post heat treatment of 350 C. to 450 C. for 30 minutes or more. Elements prepared from this material have improved speeds compared to the yellow orthorhombic lead monoxide or red tetragonal lead monoxide prepared by other processes. Care must be taken to carry out the conversion in the absence of impurities such as anionic forms of silicon, germanium, phosphorus, arsenic, antimony, selenium, tellurium, molybdenum, tungsten, etc., e.g., silicate, germanate, phosphate, metaphosphate, arsenate, antimonate, selenate, tellurate, molybdate, tungstate, etc, These impurities have been found to inhibit the conversion and the resultant product does not have improved speeds. Thus, the selection of a reaction vessel and reactants is critical and somewhat limited.

It is, therefore, an object of this invention to provide a novel class of photoconductors having high X-ray sensitivity when electrically charged.

It is another object to provide novel photoconductorcontaining compositions which exhibit high electrical speeds when exposed to X-radiation.

It is also an object to provide novel X-ray sensitive photoconductor-containing compositions which can be positively and negatively charged.

It is another object to provide novel electroradiographic elements having high speed characteristics when subjected to X-radiation.

It is a further object of this invention to provide an electroradiographic process for producing images using the novel elements of this invention.

These and other Objects of this invention are accomplished with electroradiographic elements having coated thereon X-ray sensitive photoconductive compositions containing, as the photoconductor, particulate tetragonal lead monoxide prepared by heating particulate orthorhombic lead monoxide in an aqueous solution containing carbonate ions and thereafter subjecting the resulting tetragonal lead monoxide to a heat treatment. It has been found that the presence of the carbonate ion during the conversion permits the transition from the orthorhombic to the tetragonal form even in the presence of such impurities as the silicate ion. The particulate tetragonal lead monoxide prepared in this manner has excellent photoconducting properties when used as a photoconductor in xeroradiographic elements. The elements have high speeds when subjected to X-racliation, i.e., electromagnetic radiation having a wavelength of from about 0.1 to about 100 angstroms. Higher speeds are obtainable with these elements than those containing other forms of lead monoxide as the photoconductor, e.g., orthorhombic lead monoxide. Also, when compared to elements containing tetragonal lead monoxide prepared by other processes, the speeds obtained are much greater for exposures in the X-ray region.

The tetragonal lead monoxide useful in this invention is prepared by heating a suspension of particulate Orthorhombic lead monoxide in an aqueous solution containing carbonate ions for a period sutficient to convert at least of the orthorhombic to the tetragonal form. While useful speeds are attainable from 80% conversion,

higher speeds are achieved with higher conversions. The time for the conversion ranges from about 15 minutes to about 3 hours. The conversion can be carried out by heating the slurry generally from about 50 C. to about 200 C. and preferably from 80 C. to 125 C. The conversion pressure generally ranges from about 0.1 atmospheres to about 50 suggested atmospheres and is preferably atmospheric pressure. The Weight ratio of aqueous solution to orthorhombic lead monoxide used for the conversion ranges from about 0.1 part of solution for each part of orthorhombic lead monoxide to about 10.0 parts of solution to each part of orthorhombic lead monoxide. The carbonate ion can be furnished by carbon dioxide or any compound having a carbonate anion such as sodium carbonate, ammonium carbonate, lead carbonate, potassium carbonate, etc. While trace amounts of the carbonate ion in solution are sufiicient, it is preferable to employ a solution which is at least about 0.001 molar with respect to the carbonate ion. The maximum concentration can be widely varied but generally solutions up to about 1.0 molar are suitable.

After the conversion is completed, the solids are removed and washed with distilled water and optionally additional wash liquids such as methyl alcohol, trichlorotrifluoroethane, etc. The resulting particulate red tetragonal lead monoxide is then dried at a temperature from about 50 C. to about 150 C. to remove residual wash liquids. The dried particulate material is subjected to a post heat treatment in an inert atmosphere (i.e., an atmosphere not reactive with tetragonal lead monoxide) such as argon, helium, neon, nitrogen, etc., at a temperature from about 350 C. to about 500 C. for a period sufficient to remove residual impurities. The period for such treatment is generally 30 minutes or more. The average particle size (diameter) of the final red tetragonal lead monoxide varies,

but more generally ranges from about 0.25 micron to about microns.

Electroradiographic elements can be prepared with the particulate tetragonal lead monoxide of the invention by blending a dispersion of the photoconductive material with a binder and coating the photoconductor-containing material on a suitable conducting support.

Preferred binders for use in preparing the present photoconductive layers are filming-forming, hydrophobic polymeric binders having fairly high dielectric strength which are good electrically insulating film-forming vehicles. Materials of this type comprise styrene-butadiene copolymers; silicone resins; styrene-alkyd resins; silicone-alkyd resins; soya-alkyd resins; poly(vinyl chloride); poly(vinylidene chloride); vinylidene chloride-acrylonitrile copolymers; poly(vinyl acetate); vinyl acetate-vinyl chloride copolymers; poly(vinyl acetals), such as poly(viny1 butyral); polyacrylic and methacrylic esters, such as po1y(methylmethacrylate poly (n-butylmethacrylate poly (isobutylmethacrylate) etc.; polystyrene; nitrated polystyrene; polymethylstyrene; isobutylene polymers; polyesters, such as poly(ethylenealkaryloxyalkylene terephthalate); phenolformaldehyde resins; ketone resins; polyamides; polycarbonates; polythiocarbonates; poly(ethyleneglycol-co-bishydroxyethoxyphenyl propane terephthalate); copolymers of vinyl haloarylates and vinyl acetate such as poly(vinylm-bromobenzoate-co-vinylacetate); etc. Methods of making resins of this type have been described in the prior art, for example, styrene-alkyd resins can be prepared according to the method described in US. Pats. 2,361,019 and 2,258,423. Suitable resins ofthe type contemplated for use in the photoconductive layers of the invention are sold under such trade names as Vitel PE-101, Cymac, Pliolite S-5, Piccopale 100, Saran F-220, Lexan 105 and Lexan 145. Other types of binders which can be used in the photoconductive layers of the invention include such materials as paraflin, mineral waxes, etc.

Solvents of choice for preparing coating compositions of the present invention can include a number of solvents such as benzene, toluene, acetone, 2-butanone, chlorinated hydrocarbons, e.g., methylene chloride, ethylene chloride,

4 etc., ethers, e.g., tetrahydrofuran, or mixtures of these solvents, etc.

In preparing the coating composition useful results are obtained where the photoconductor substance is present in an amount equal to at least about 1 Weight percent of the coating composition. The upper limit in the amount of photoconductor substance present can be widely varied in accordance with usual practice. More generally, from 1 to 12.5 parts by weight of photoconductor for each part by weight of binder in the final composition is used. A preferred weight range in the final composition is 1.5 to about 7.5 parts by weight of photoconductor for each part by weight of binder.

Coating thicknesses of the photoconductive composition on a support can vary widely. More generally, a coating in the range of about 0.001 inch to about 0.10 inch before drying is useful for the practice of this invention. The preferred range of coating thickness is in the range from about 0.002 inch to about 0.02 inch before drying although useful results can be obtained outside of this range.

Suitable supporting materials for coating the photoconductive layers of the present invention can include any of a wide variety of electrically conducting supports, for example, paper (at a relative humidity above 20 percent); aluminum-paper laminates; metal foil such as aluminum foil, zinc foil, etc.; metal plates, such as aluminum, copper, Zinc, brass, and galvanized plates; vapor deposited metal layers such as silver, nickel or aluminum and the like on paper and resin film supports. An especially useful conducting support can be prepared by coating a resin film support material such as poly(ethylene terephthalate), cellulose acetate, etc., with a layer containing a semiconductor dispersed in a resin. Such conducting layers both with and without insulating barrier layers are described in US. Pat. 3,245,833. Likewise, a suitable conducting coating can be prepared from the sodium salt of a carboxyester lactone of a maleic anhydride-vinyl acetate interpolymer. Such kinds of conducting layers and methods for their optimum preparation and use are disclosed in US. 3,007,901 and 3,267,807.

The elements of the present invention can be employed in a xeroradiographic process. In a process of this type the electroradiographic element is given a blanket electrostatic charge by placing the same under a corona discharge which serves to give a uniform charge to the surface of the photoconductive layer. This charge is retained by the layer owing to the substantial insulating property of the layer, that is, the low conductivity of the layer in the absence of X-radiation. The electrostatic charge formed on the surface of the photoconducting layer is then selectively dissipated from the surface of the layer by exposure to a pattern of X-rays which is to be reproduced so that the X-rays discharge the irradiated areas by photoconduction. By exposure of the surface in this manner, a charged pattern is created by virtue of the fact that the X-rays cause the charge to be conducted away in proportion to the intensity of the irradiation in a particular area. The charge pattern remaining after exposure is then developed, i.e., rendered visible, by treatment with a medium comprising electrostatically attractable particles having optical density. The developing electrostatically attractable particles can be in the form of a dust, e.g., powder, pigment in a resinous carrier, i.e., toner, or a liquid developer may be used in which the developing particles are carried in an electrically insulating liquid carrier. Methods of development of this type are widely known and have been disclosed in US. Pat. 2,297,691 and in Australian Pat. 212,315. In processes of electroradiographic reproduction such as in xeroradiography, by selecting a developing particle which has as one of its components, a low-melting resin, it is possible to treat the developed photoconductive material with heat and cause the powder to adhere permanently to the surface of the photoconductive layer. In other cases, a

transfer of the image formed on the photoconductive layer can be made to a second support such as paper, which would then become the final print. Techniques of the type indicated are well known in the art and have been described in US. Pats. 2,297,691 and 2,551,582 and in RCA Review, vol. 15 (1954), pages 469-484, for example.

Additionally, the electrostatic charge comprising the latent image which is produced on the surface of the photoconductive element after exposure can be transferred to a receiving sheet and developed there. The charging and exposing of the photoconductive element and the transfer of the latent image can occur simultaneously as described in Walkup US. Pat. 2,825,814.

The electroradiographic materials described herein are particularly responsive to X-radiation, i.e., radiation having a wavelength from about 0.1 angstrom to about 100 angstroms and are useful in various types of xeroradiographic systems.

The invention is further illustrated by the following examples which include preferred embodiments thereof.

EXAMPLE 1 500 grams of particulate yellow orthorhombic lead monoxide designated as Evans Fumed Litharge (Evans Lead Corp), having a diameter ranging from 0.25 to microns, and 10 grams of ammonium carbonate are added to 500 cc. of boiling distilled water. The mixture is allowed to boil for 30 minutes. After filtering, the solids are washed once with distilled water, twice with methyl alcohol and twice with Freon 113 (trichlorotrifluoroethane). After drying at 110 C. for several hours, the solids are fired in a quartz boat in a furnace at 400 C. for one hour in nitrogen. X-ray diffraction indicates that all of the yellow orthorhombic lead monoxide is converted to the particulate red tetragonal form by the above procedure. The particle size of the tetragonal material ranges from 0.25 micron to 10.0 microns.

EXAMPLE 2 Example 1 is repeated except 20 grams of sodium silicate are added to the aqueous solution containing ammonium carbonate. X-ray diffraction indicates that all of the particulate yellow orthorhombic lead monoxide is converted to the particulate red tetragonal form.

EXAMPLE 3 Example 2 is repeated except that the amonium carbonate is omitted. According to X-ray dilfraction, none of the orthorhombic material is converted to the tetragonal form.

EXAMPLE 4 Example 2 is repeated except the ammonium carbonate is replaced by carbon dioxide. The carbon dioxide is bubbled through the slurry during the conversion at approximately 40 mL/min. All of the particulate orthorhombic lead monoxide is converted to the particulate tetragonal form.

EXAMPLE 5 Example 2 is repeated except the ammonium carbonate is replaced by 20 grams of sodium carbonate. All of the particulate orthorhombic lead monoxide is converted to particulate red tetragonal lead monoxide.

EXAMPLE 6 About 50 parts by weight of particulate orthorhombic lead monoxide are boiled for one hour in 148 parts by weight of a 12 molar aqueous solution of sodium hydroxide containing 20 parts by weight of sodium silicate. The resulting crystals are separated by repeated decantation and washing with distilled Water followed by washing with absolute methanol. X-ray analysis shows that none of the orthorhombic material is converted to the tetragonal form.

6 EXAMPLE 7 A coating composition is prepared from the tetragonal lead monoxide of Example 4 by milling the following composition for 24 hours with agate balls:

(a) 40 grams tetragonal lead monoxide,

(b) 20 grams Pliolite S7 (trademark of Goodyear Tire and Rubber Co. for a 70:30 styrene-butadiene copolymer), and

(c) 86.7 grams toluene.

The resulting composition is coated at 0.010 inch wetthickness on an aluminum foil paper laminate directly on the aluminum. The coating block is maintained at a temperature of F. until dry. This electroradiographic element is charged under a negative corona source until the surface potential, as measured by an electrometer probe, reaches about 450 volts. The potential of the surface is measured just prior to exposure (V and immediately after exposure (V) to a 900 mr. source (generated by 50 kv. at 25 ma). The exposure causes a reduction of the surface potential. The speed of the element is expressed as the change in the surface potential divided by the surface potential immediately prior to exposure multiplied by 100, i.e.,

Speed:

The above element has a negative speed of 57.

EXAMPLE 8 Particulate yellow orthorhombic monoxide obtained from the Evans Lead Corp. having a particle size ranging from 0.25 to 10 microns in diameter is heat treated at 300 C. for one hour in air. Electroradiographic elements are prepared from this material and tested in the manner described in Example 7, using instead a positive charging polarity. These elements are found to have a positive speed of 4.5.

In comparing Examples 1 through 6 it is seen that when such impurities are present as the silicate ion, orthodrombic lead monoxide does not convert to the tetragonal form unless the carbonate ion is present. Examples 7 through 9 show that the particulate red tetragonal lead monoxide is more sensitive to X-radiation when used in electroradiographic elements than the particulate yellow orthorhombic form. Thus, it is desirable to be able to easily convert the yellow form to the red form even in the presence of such impurities as the silicate ion.

EXAMPLE 10 Coating dopes are prepared in the manner described in Example 7 using the materials set forth therein. The red tetragonal lead monoxide used is prepared by the method described in Examples 1, 2, 4, and 5. In a darkened room, the surface of each of the elements so prepared is charged to a potential of about +450 volts under a corona charger. The elements are then exposed to a pattern of X-rays from a 240 mr. source. The resulting electrostatic latent images are developed in the usual manner by cascading over the surfaces of the layers a mixture of negatively charged black thermoplastic toner particles on glass beads functioning as carriers for the toner particles. A good reproduction of the pattern results in each instance. Similar results are obtained using a liquid developer.

The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be etfected within the spirit and scope of the invention.

I claim:

1. An electroradiographic element useful in xeroradiography comprising a support having coated thereon an X-ray sensitive composition comprising an electrically insulating film-forming resin binder having dispersed therein a particulate tetragonal lead monoxide photoconductor prepared by a process comprising:

(a) heating particulate orthorhombic lead monoxide in an aqueous solution containing carbonate ions for a period sufficient to convert at least 80% of said orthorhombic lead monoxide to tetragonal lead monoxide,

(b) separating the resulting particulate tetragonal lead monoxide, and

(c) heating the tetragonal lead monoxide thus separated at a temperature of about 350 C. to about 500 C. in an atmosphere inert to tetragonal lead monoxide for a period suflicient to remove impurities.

2. The electroradiographic element as defined in claim 1 wherein the binder is a copolymer of styrene and butadiene.

3. The electroradiographic element as defined in claim 1 wherein the ratio of photoconductor to binder is at least 1:1 by weight.

4. The electroradiographic element as defined in claim 1 wherein the aqueous solution comprises water and a compound selected from the group consisting of carbon dioxide, ammonium carbonate, lead carbonate and sodium carbonate.

5. The electroradiographic element of claim 1 wherein the aqueous solution is at least 0.001 molar with respect to the carbonate ion.

6. The electroradiographic element of claim 1 wherein the particulate orthorhombic lead monoxide is converted to particulate tetragonal lead monoxide by heating in an aqueous solution containing carbonate ions for at least minutes.

7. The electroradiographic element of claim 1 wherein the separated particulate tetragonal lead monoxide is heated at a temperature of about 350 C. to about 500 C. in an inert atmosphere for at least 30 minutes.

8. The electroradiographic element of claim 1 wherein the separated particulate tetragonal lead monoxide is.

heated at a temperature of about 350 C. to about 500 C. in a nitrogen atmosphere for a period sufficient to remove impurities.

9. An electroradiographic element useful in xeroradiography comprising a support having coated thereon an X-ray sensitive composition comprising a styrene-buta diene copolymeric binder having dispersed therein a particulate tetragonal lead monoxide photoconductor in a ratio of 1 part binder for each 1 to 12.5 parts by weight of photoconductor prepared by a process comprising:

(a) heating particulate orthorhombic lead monoxide in an aqueous solution containing carbonate ions for at least 15 minutes to form particulate tetragonal lead monoxide solid,

(b) separating the resulting particulate tetragonal lead monoxide, and

(c) heating the tetragonal lead monoxide thus separated at a temperature of about 350 C. to about 500 C. in a nitrogen atmosphere for at least 30 minutes.

10. A process for preparing an electroradiographic element useful in xeroradiography comprising the steps of:

(a) heating particulate orthorhombic lead monoxide in an aqueous solution containing carbonate ions for a period sufficient to convert at least 80% of said orthorhombic lead monoxide to tetragonal lead monoxide,

(b) separating the resulting particulate tetragonal lead monoxide,

(c) heating the tetragonal lead monoxide thus separated at a temperature of about 350 C. to about 500 C. in an atmosphere inert to tetragonal lead monoxide,

(d) mixing the particulate tetragonal lead monoxide with an insulating binder, and

(e) coating the resultant mixture on a conducting support.

11. The pocess of claim 10 wherein the insulating binder is a copolymer of butadiene and styrene.

12. The process of claim 10 wherein the inert atmosphere comprises nitrogen.

13. The process of claim 10 wherein the weight ratio of tetragonal lead monoxide to binder is at least 1:1.

14. The process of claim 10 wherein the aqueous solution of (a) comprises water and a compound selected from the group consisting of carbon dioxide, ammonium carbonate, lead carbonate and sodium carbonate.

15. The process of claim 10 wherein the aqueous solution of (a) is at least 0.001 molar with respect to the carbonate ion.

16. The process of claim 10 wherein the particulate orthorhombic lead monoxide of (a) is converted to particulate tetragonal lead monoxide by heating in an aqueous solution containing carbonate ions for at least 15 minutes.

17. The process of claim 10 wherein the separated particulate tetragonal lead monoxide is heated at a temperature of about 350 C. to about 500 C. in an inert atmosphere for at least 30 minutes.

1 8. The process of claim 10 wherein the separated particulate tetragonal lead monoxide is heated at a temperature of about 350 C. to about 500 C. in a nitrogen atmosphere for a period sufficient to remove impurities.

19. A process for preparing an electroradiographic element useful in xeroradiography comprising the steps of:

(a) heating particulate orthorhombic lead monoxide in an aqueous solution containing carbonate ions at a temperature of C. to C. for at least 15 minutes to form tetragonal lead monoxide solid,

(b) separating the resulting particulate tetragonal lead monoxide,

(c) heating the tetragonal lead monoxide thus separated at a temperature of about 350 C. to about 500 C. in a nitrogen atmosphere for at least 30 minutes,

(d) mixing the particulate tetragonal lead monoxide with a styrene-butadiene copolymeric binder, and

(e) coating the resultant mixture on a conducting support.

20. In the method of forming an electrostatic latent image comprising electric charges on an insulating receiving surface said method comprising:

(a) positioning an insulating receiving surface in virtual contact with the surface of a photoconductive insulating layer disposed on a conductive backing support, said insulating receiving surface being disposed and positioned in face to face relationship with the surface of the photoconductive insulating layer,

(b) applying a potential across said photoconductive insulating layer and insulating receiving surface while (0) simultaneously exposing the photoconductive insulating layer to a pattern of X-radiation, and

(d) while the potential continues to be applied, separating the insulating surface from the photoconductive insulating layer, the improvement characterized in that the photoconductive insulating layer comprises particulate tetragonal lead monoxide prepared by (e) heating particulate orthorhombic lead monoxide in an aqueous solution containing carbonate ions for a period suflicient to convert at least 80% of said orthorhombic lead monoxide to tetragonal lead monoxide,

(f) separating the resulting particulate tetragonal lead monoxide, and

9 10 (g) heating the tetragonal lead monoxide thus sepa- 3,453,141 7/1969 Anolick et a1. 117201 rated at a temperature of about 350 C. to about 3,468,705 9/1969 Schottmiller et a1. 117201 500 C. in an atmosphere inert t0 tetragonal lead monoxide for a period sufficient to remove impurities. BORCHELT, Prlmary er References Cited 5 A. L. BIRCH, Assistant Examiner UNITED STATES PATENTS US. Cl. X.R.

3,008,825 11/1961 Van Dorn et a1. 961 96-1; 117201; 252-501 3,266,932 8/1966 Anolick 117201 

